1. Technical Field
The invention generally relates to power supplies and, more particularly, to a power supply having an opposed current converter that performs power factor correction.
2. Related Art
Power supplies are utilized as a source of power in many electrical devices including most devices having electronic circuits. A power supply may utilize input power from a single phase or multiple phase alternating current source to produce output power. The output power may be produced at one or more predetermined voltages with a determined range of output current. The output power may be alternating current (AC) or direct current (DC) of almost any magnitude depending on the load the power supply is serving.
Some power supplies and associated electrical device loads may be classified as non-linear power electronic loads. Such non-linear power electronic loads typically include rectifier/capacitor input stages that are characterized by an undesirably low power factor due to excessive load current harmonics. Load current harmonics cause an increase in the magnitude of RMS current supplied to such a non-linear power electronic load. Load current harmonics result in a reduction in power factor because they do not provide useful power to the non-linear power electronic load.
Multiple kilowatt non-linear power electronic loads, such as a high power audio amplifier or a magnetic resonance imaging gradient amplifier, place significant current demands on a source of input power. A power feed from a source of input power may be supplied to a load from a circuit breaker with limited current carrying capacity. For example, a power feed from a source of input power that is a single phase power distribution system may be supplied from a circuit breaker that is rated for fifteen amps of sustained RMS current at near unity power factor. When a load with a low power factor is present, the RMS current requirement is higher, and the circuit breaker may open the power feed even though the load is not utilizing substantial power.
Power factor correction (PFC) may be used to decrease the magnitude of additional RMS current resulting from harmonics. Power factor correction may involve working to maintain the current drawn from an AC source in phase and identical in form with the voltage drawn from the AC source. For non-linear power electronic loads, there are passive and active power factor correction approaches. Passive approaches include series inductor filters and resonant filters. Active approaches include boost derived converters and other switch mode based systems.
In general, boost derived converters operate with switching frequencies higher than the frequency of the source of input power (typically 50-60 Hz) to control the shape of the input current waveform. The operating frequency of boost derived converters may result in undesirably high current ripple frequencies. In addition to power factor correction, boost derived converters that are referred to as universal input boost converters have the capability to accept a range of input voltages such as 100VAC nominal (Japan), 120VAC nominal (United States) and 230VAC nominal (Europe). Boost derived converters may also provide voltage regulation of the output voltage of the converter.
Some boost derived converters operate with PFC in a discontinuous conduction mode (DCM). To minimize ripple current associated with such switch mode operation, some boost derived converters operate with an interleaved configuration. The interleaved configuration involves multiple switches that are operated sequentially during a switching period to increase ripple frequency while reducing ripple magnitude. The reduction in ripple magnitude further decreases undesirable line currents and therefore improves power factor.
The reduced magnitude of ripple current, however, still creates undesirable load currents. In addition, significant power losses are experienced within known boost derived converters due to the number of stages the power must be processed through. A boost derived converter may include a first stage that is a bridge rectifier, a second stage that is a DC to DC boost converter and a third stage that is a DC to DC power converter with galvanic isolation. Significantly increased power losses also occur in known boost derived converters during conditions of low supply voltage due to high input currents. Some boost derived converters also must limit the size of boost inductors included in the boost derived converter due to the inability to quickly magnetize and demagnetize the inductors. Limiting the size of the boost inductors may result in increased ripple current, to avoid distorting the input current over time (dI/dt) during periods of low input voltage.
Therefore a need exists for a power factor correcting power supply with lower harmonics, greater power efficiency, and minimization of the ripple current.